Thermostatic control device with heat motor operated step open diaphragm valve

ABSTRACT

A control device for burner apparatus having a differential pressure operated diaphragm valve controlled by an internal bleed system which is subject to pressure regulation and to on-off control by sequentially operated bypass and bleed valves which are actuated by a heat motor operator. Initial actuation of the heat motor operator opens the bleed valve to provide a step input at a reduced rate under pressure regulation to the main burner for assuring good ignition without &#39;&#39;&#39;&#39;roll-out.&#39;&#39;&#39;&#39; Subsequent control of the bypass valve enables flow modulation, and full input is delayed until the bypass valve is fully closed.

United States Patent Haskins et a1.

[54] THERMOSTATIC CONTROL DEVICE WITH HEAT MOTOR OPERATED STEP OPEN DIAPHRAGM VALVE Inventors: Lauren D. Haskins, Long Beach;

Samuel T. Kelly, Torrance, both of Calif.

Assignee: Robertshaw Controls Company,

Richmond, Va.

Filed: June 3, 1970 Appl. No.: 43,013

References Cited UNITED STATES PATENTS 2,724,409 11/1955 Coffey ..236/1EX 3,155,316 11/1964 Bransonetal ..236/48 1451 Aug. 22, 1972 Pr imary Examiner--William Ehwayner Att0rney--Auzville .lackson,;Jr., Robert L'. Marben and Anthony A. O'Brien '57 ABSTRACT A control device for burner apparatus having a differential pressure operated diaphragm valve con trolled by an internal bleed systemiwhich is subject to pressure regulation and to on-off control by sequentially operated bypass and bleed valves which are actuated by a heat motor operator. Initial actuation of the heat motor operator opens the bleed valve to provide a step input at a reduced rate under pressure regulation to the main burner for assuring good ignition without roll-out. Subsequent control of the bypass valve enables flow modulation, and full input is delayed until the bypass valve is fully closed. A

l 1 Claims, 6 Drawing Figures ASSEMBLY 22 GAS 2 .PATENTEDAUBZZIHYZ 3.685732 sum 1 or z THERMOSTAT /4 ASSEMBLY uvvavvms LAUREN 0. HASK/NS SAMUEL 7.- KELLY 28 By mum 4 My W- m.

A TORNE Y v PATENTE'lllumzzI912 4 3,685,732

SHEET 2 OF 2 BLEED VALVE BY PASS OPEN CLOSED BYPASS OPENING FULL v I OUTLET I PRESSURE STEP OFF

ON 4 FULL on DEENERBIZED OFF TIME FULL I q BYPASS VALVE- OUTLET MODULATION PRESSURE BAND 4STEP- &

OFF

am y OFF TIME FULL Q cam) STAGE) OUTLET I PR&SURE

STEP v s (1ST STAGE) OFF TIME

INVENTORS LAUREN '0. HA$K/N$ SAMUEL 7.- KELLY A T TORNE V THERMOSTATIC CONTROL DEVICE WITH HEAT MOTOR OPERATED STEP OPEN DIAPHRAGM VALVE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention pertains generally to the control fuel flow to burner apparatus, and in particular, to a combination control device wherein the fuel flow is subject to step opening and modulation control by a bleed line flow.

known prior art devices, however, often require complex operating mechanisms, contain a duplicity .of operating components, and fail to provide a significant delay upon step opening. In addition, such conventional devices are not adaptable to staging operation or modulation under the control of solid state thermostats.

While the prior art control devices have served the purpose for their particular installations, they have not proven entirely satisfactory under all conditions of operation for the reasons enumerated above, among others, and due to their relatively high construction and maintenance costs.

SUMMARY OF THE INVENTION The present invention is summarized in that a control device is provided with a casing having an inlet and an outlet and a differential pressure operated diaphragm valve mounted in the casing between the inlet and the outlet and operatively movable between a plurality of flow positions, a bleed chamber controlling an operating pressure on the diaphragm valve means and having first and second ports, a primary bleed flow passage between the inlet and the bleed chamber, a secondary bleed flow passage between the inlet and the first port, a bleed line between the second port and the outlet, a valving arrangement in the bleed chamber cooperating with the first and second ports to control the operating pressure on the diaphragm valve and having'an off position wherein the first port is open and the second port is closed, an intermediate position wherein the first and second ports are open, and an on position wherein the first port is closed and the second port is open, and a heat motor operatively connected with the valving arrangement to effect delayed movement of the valving arrangement between the off, intermediate and on positions whereby the diaphragm valve is sequentially moved between an off position, a step flow position and a full flow position.

An object of the present invention is to construct a combination control device to provide a step flow prior to the establishment of normal full flow.

The present invention has another object in that a flow control system is operated by a bleed line flow to provide step opening and flow modulation of a main valve.

Another object of this invention is to construct a combination control device having a bleed line valving arrangement controlled by a relatively slow acting heat motor operator.

The present invention has a further object in that a restricted-flow bleed passage is bypassed by a valved parallel bleed passage controlled by a heat motor operator in response to ambient temperature variations.

The present invention is advantageous over prior art devices in the provision of a combination flow control device having step opening, flow modulation, increased versatility, simplicity in design and construction, and reduced cost.

Other objects and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a flow control system embodying a thermostatic control device according to the present invention and showing the various components in their off position;

FIG. 2 is a partial schematic diagram of a detail of the flow control system of FIG. 1 with the operating components in a first operative position; 7

FIG. 3 is a partial schematic diagram similar to FIG. 2 with the various components shown in a second operative. position;

FIG. 4 is an outlet pressure vs. time curve illustrating the operation of the thermostatic control device of the present invention under the control of a bimetal thermostat;

FIG. 5 is a curve similar to FIG. 4 illustrating the operation of the present invention under the control of a modulating thermostat; and

FIG. 6 is a curve similar to FIG. 4 illustrating the operation of the present invention under the control of a staging thermostat.

DESCRIPTION OF THE PREFERRED EMBODIMENT While the present invention is applicable to various types of controls for heating and/or cooling apparatus, it will be described in connection with burner ap paratus of the heating type. Such apparatus, as shown in FIG. 1, includes a main burner 10 and a pilot burner 12 located in igniting proximity thereto and connected to a fuel source such as a gas supply. As is well known in the art, a thermoelectric pilot safety valve (not shown) may be included in the gas supply line. The main burner 10 is part of a furnace (not shown) supplying heat to a particular area in which is located a thermostat assembly 14 which may be of any suitable type such as a bimetal switch, a modulating thermostat circuit, or an electro-mechanical two-stage thermostat, depending upon the operational characteristics desired, as will be more fully described below. The thermostat assembly 14 is connected between a suitable source of electricity, represented by lines 16, and pair of terminals 18 which are mounted upon and insulated from a hollowed casing indicated generally at 20.

Casing 20 has an inlet portion 22 on one end connected to the gas supply and an outlet port 24 on an opposite end connected to the main burner 10 by a suitable conduit. The inlet and outlet ports are disposed on a common axis with a valve seat 26 intermediately located therebetween. A flow through the valve seat 26 I is controlled by a flexible diaphragm defining a main diaphragm valve 28. The periphery of the diaphragm valve 28 is clamped between adjacent sections of the casing 20, which are secured together as by cap screws (not shown). The main diaphragm valve 28 separates a hollow cavity of the casing into an inlet pressure chamber 30 and an operating pressure chamber 32. A back-up plate 34 is secured to the undersurface of diaphragm valve 28, and a coil spring 36 is mounted in compression between the bottom casing wall of the operating pressure chamber 32 and the back-up plate 34 whereby the diaphragm valve is biased towards engagement with the valve seat 26.

A bleed flow passage 38 having a flow restricting orifree 40 establishes communication between the inlet port 22 and a bleed chamber 42 which communicates with the operating pressure chamber 32 by means of a bleed passageway 44. A second bleed line passageway 46 is formed in one wall of casing 20 and has a generally cylindrical, hollow sleeve 48 secured at its upper end within passageway 46 and extending at its lower end into chamber 42. Sleeve 48 is constructed to have an inner surface contiguous with passageway 46 and a lower edge formed to provide a valve seat or port 50. Bleed flow through valve seat 50 and passageway 46 is controlled by a flexible valve member 52 which is carried upon the upper surface of an offset leg 54 of a lever arm 56 which is pivotally mounted by a hinge member 58 to a wall of chamber 42.

Parallel to the primary bleed flow passage 38 is a secondary or bypass flow passage 60 having a flow restricting orifice 62 which may be adjusted by an adjusting screw 64 mounted within the casing 20. The bleed flow passage 60 extends from the inlet port 22 through an opening 66 to an annular valve seat or port 68. Valve seat 68 is centrally formed in a bottom wall of a generally cylindrical supporting member 70 which is attached to an upper wall of the bleed chamber 42 in concentric alignment over opening 66. A bypass valve member 72 cooperates with valve seat 68 to control the flow of fuel through the secondary bleed flow passage 60 and has a valve shaft 74 which is axially aligned with and extends through valve seat 68. A spring retaining disc 76 is fixedly located adjacent the lower end of valve shaft 74, and a biasing spring 78 is maintained in compression between the lower surface of cylindrical support member 70 and the upper surface of disc 76. An actuating leg 80 for bypass valve 72 is formed upon the free end of lever arm 56 such that the rotational movement of the lever arm about pivot member 58 controls the operative position of valve member 72 with respect to the valve seat 68.

The angular position of lever arm 56 is controlled by a relatively slow acting heat motor operator which includes a thermal compensating bimetal 82 which is mounted at one end to the interior of bleed chamber 42 and has its other end connected by a suitable connecting link, such as a bolt 84, to the distal end of an active bimetal 86. The other end of bimetal 86 is rigidly mounted upon the joumaled end of lever arm 56 for imparting rotational movement thereto. A heating coil 88, which may be of any suitable construction is electrically connected with terminals 18 and is wound around or otherwise placed in heating proximity with bimetal 86 for heating the bimetal in response to operating potential received from power supply 16 under the control of the thermostat assembly 14. It should be understood, of course, that the functions of the active and compensating bimetals of the heat motor operator may be reversed by reversing the high-low expansion sides of each bimetal and winding the heater coil around the other bimetal.

A bleed flow from bleed chamber 42 through bleed line 46 is controlled by a pressure regulator 90, and an outlet bleed line 92 cooperates with an outlet port 94 to establish communication between the outlet side of the regulator 90 and the main outlet port 24. It is noted that the pressure regulator may be of any suitable type such as that shown in FlG. l which includes a regulating valve having an axially elongated valve face controlling the bleed line flow from passage 46 into a regulating chamber which communicates with the bleed line outlet passage 92. A movable wall of the regulating chamber is defined by a flexible diaphragm which has one side attached to the regulating valve and its opposite side subject to atmospheric pressure by a suitable vent in the regulator housing. A high-rate coil spring 96 exerts a biasing force upon such opposite side of the flexible diaphragm, and the biasing force is adjustable by a set screw 98 in thecover member on the top of the regulator housing.

In the following description of a sequence of operation, it is assumed that the main burner 10 is located in a furnace (not shown) supplying heat to a space in which the thermostat 14 is located. FIG. 1 shows the off condition of the control elements indicating that the heat requirements for such space have been satisfied. Referring to FIG. 1, when thermostat assembly 14 is off, the heating coil 88 receives no electrical energization and bimetal 86 assumes its relaxed or cold position as shown. As a result, lever arm 56 is in its off position causing closure of valve member 52 against port 50 so that there is no bleed flow from the bleed chamber 42; at this same time, leg of lever arm 56 moves valve shaft 74 up against the force exerted by biasing spring 78 to lift bypass valve member 72 away from port 68 so that there is communication between the bleed chamber 42 and both the primary and secondary bleed passages 38 and 60, respectively. Since the bleed passage 44 is always open between bleed chamber 42 and operating pressure chamber 32, the operating pressure chamber 32 will be subject to full inlet pressure when no demand for burner operation exists. The pressure exerted upon both sides of main diaphragm valve 28 will therefore be equalized such that the coil spring 36 biases the main diaphragm valve 28 to its closed position against the main valve seat 26.

When a demand for heat is signaled, the thermostat assembly 14 responds accordingly by applying operating potential through terminals'l8 to heating coil 88 which thereafter begins to heat bimetal 86. It should be recalled at this point that the pressure in bleed chamber 42 prior to a demand for heat is equal to inlet pressure, while the pressure downstream of port 50 is equal to outlet pressure. Since main valve 28 is closed at this time, the outlet pressure is lower than the inlet pressure so that a pressure differential exists across valve member 52 in a direction which tends to maintain the valve member closed against valve seat or port 50. Thus, upon initial energization of heating coil 88 in above described pressure differential is overcome by the movement of bimetal 86. As illustrated in FIG. 2, the heating of bimetal 86 causes its distal end to try to move upwardly; however, since such end is connected with thermal compensating bimetal 82 by bolt 84, it is prevented from so moving, thereby causing rotation of lever arm 56 in a counterclockwise direction about pivot member 58. It can be seen that the energization of the active bimetal will initially cause compensating bimetal 82 and lever arm 56 to flex slightly until a sufficient force is produced at valve member 52 to snap the Referring to FIG. 3, after the step opening delay time has fully elapsed, bimetal 86 will have been fully heated by heating coil 88 so as to curve up and cause full counterclockwise rotation of lever arm 56. In this manner, leg 80 of lever arm 56 moves down permitting bypass valve 72 to close against valve seat 68 under the force exerted by biasing spring 78. Leg 80 thereafter moves away from the lower end of valve shaft 74 so as to asvalve member away from the valve seat 50 to establish a bleed flow from the bleed chamber 42 through bleed line 46 and pressure regulator 90 and thence through outlet passage 92 and port 94 to the main outlet port 24. While the initial counterclockwise movement of lever 56 permits valve 72 to move slightly toward its seat 68 under the force of spring 78, the port 68 remains open such that both the primary and secondary bleed passages 38 and 60, respectively, initially remain in communication between the bleedchamber 42 and the inlet 22. Since boththe primary and secondary bleed passages are open when the valve 52 snaps open,

the pressure in bleed chamber 42, and consequently that in operating pressure chamber 32, rapidly drops to an intermediate value which is dependent upon the bleed flow through. the secondary or bypass bleed passage 60 and the regulation characteristics of pressure regulator 90. Under these conditions, the main valve 28 opens to an intermediate position due to the reduction in pressure in the operating pressure chamber 32 from full inlet pressure to an intermediate pressure between full inlet pressure and that required for full regulated outlet pressure operation. Thus, upon initial energization, the thermostatic control of the present invention rapidly opens to establish a low or stepped flow rate for feeding main burner '10. It should be understood that the step outlet pressure can be adjusted by resetting the bleed flow through the secondary bleed passageway 60 by adjusting orifice 62 via the adjusting screw 64; i.e., by opening orifice 62, the step outlet pressure will be reduced, and similarly, by closing the orifice 62, the step outlet pressure will be increased.

The step open position (FIG. 2) of the main diaphragm valve 28 supplies a flow of fuel to the main burner 10 at a low rate sufficient to permit ignition and yet not great enough to oversupply the burner 10 which would cause roll-out of the entire flame. Once the main burner 10 is ignited with a low flame, the bumer may be supplied with a full rate of flow in accordance with burner capacity. It is thus apparent that there is a need for a slight time delay after the time the initial low flame is established before supplying a full rate of flow. This time delay is accomplished in accordance with the present invention by the bimetal 86 in conjunction with heating element 88; The bimetal 86 is designed to be relatively slow acting such that movement from its cold.

or relaxed position, as shownin FIG. 1, to the stepped position in FIG. 2 and thence to the fully open position illustrated in FIG. 3 occurs over a time interval which is sufiiciently long to permit the main burner 10 to become fully ignited with a low flame prior to the full opening of main diaphragm valve 28.

sure complete closure of port 68 by valve member 72. Thus, when lever arm 56 has moved to its full counterclockwise position, the secondary bleed passageway is completely closed so that bleed chamber 42 communicates with inlet port 22 through the primary bleed passageway 38 only. Since port 50 remains fully open when lever arm 56 rotates counterclockwisenhe outlet passageway through bleed line 46, regulator 90, outlet passageway 92, port 94 and mainoutlet 24 remains open such that the pressure within bleed chamber 42 drops below its step or intermediate value causing the pressure in operating pressure chamber 32 to be reduced correspondingly. The pressure drop inoperating pressure chamber 32 thus causes-further movement of main diaphragm valve 28 away from its valve seat 26 to establish full flow from the gas supply to the main burner 10. When bypass valve 72 is closed and the main valve 28 is fully open, bleed gas can only enter bleed chamber 42 through orifice 40 of primary bleed passage'38. Since the restricting orifice 40 is relatively small, only a small throughput of bleed gas flows through the pressure regulator 90 which senses the high outlet pressure and accordingly regulates the gas flow about the regulator set point.

When the demand for heat ceases, the thermostat assembly 14 will interrupt the flow of energizing potential to heating coil 88 so as to permit bimetal 86 to begin to cool. As the bimetal cools, the lever arm 56 will begin to rotate clockwise causing valve 72 to gradually open and increase the pressure in bleed chamber 42 for placing the main diaphragm valve 28 in its intermediate or stepped flow position as illustrated in FIG. 2. The resulting increased throughput of bleed gas through chamber 42 thus tends to close main diaphragm valve 28 thereby lowering the control outlet pressure. The pressure regulator 90 senses the reduced outlet pressure and opens farther in an attempt to maintain the set outlet pressure. In its more open position, the pressure regulator 90 controls at a lower outlet pressure since the regulator spring 96 becomes partially relaxed as it follows the regulator valve. Furthermore, the high-rate regulator spring 96 and the axially elongated regulator valve face tend to exaggerate the lowering of the outlet pressure control point with a relatively small change in bleed gas throughput.

The main diaphragm valve 28 will remain in its intermediate position until lever arm 56 rotates further to close valve member 52 against port 50 as shown in FIG. 1. As before, the pressure differential across valve 52 tends to snap the valve closed against port 50 when lever arm 56 brings the valve close to its valve seat such that the pressure within bleed chamber 42 is rapidly increased once again to the inlet pressure so as to equalize the main diaphragm valve 28. This enables spring 36 to lift valve 28 against its valve seat 26 to cut off the fuel flow to the burner 10 and cease the heating operation.

Referring now to FIG. 4, a curve of outlet pressure vs. time graphically illustrates the operation of the thermostatic control device as described above. As seen in the graph, as bimetal 86 is initially energized, valve member 52 snaps open causing the outlet pressure at main outlet port 24 to rapidly increase to the step outlet pressure level for establishing a low rate flow for main burner ignition. As the heating of bimetal 86 continues, valve member 52 moves further away from port 50, and at the same time bypass valve 72 approaches valve seat 68 and begins to close. When valve 72 begins i to close, the pressure in bleed chamber 42 begins to drop further, causing the main diaphragm valve 28 to gradually open to its full position. Thus, when the bimetal 86 is fully heated, and the secondary or bypass bleed passage 60 is closed by bypass valve 72, full outlet pressure is established for the main burner 10. Full pressure will thereafter continue until the thermostat assembly 14 deenergizes heating coil 88 permitting bimetal 86 to begin to cool. As explained above, when the bimetal cools, lever arm 56 rotates clockwise and engages valve stem 74 to begin opening bypass valve 72. As the bypass valve 72 moves away from its seat 68, the pressure in bleed chamber 42 builds up once again until its intermediate or step value is obtained. That is, the pressure in bleed chamber 42 builds up until bypass valve 72 is fully opened defining the step position of main diaphragm valve member 28. The step position will thereafter be maintained until valve member 52 snaps against port 50 to block the flow of bleed fuel from bleed chamber 42 and equalize the pressure across the main diaphragm valve 28. This closes the main valve 28 against its valve seat 26 and cuts off fuel flow to the burner.

As mentioned above, thermostat assembly 14 may take any number of suitable forms other than a bimetal operated thermostat. For example, a modulating thermostat circuit of any suitable design may be employed, so as to modulate the energization of heating coil 88 for modulating fuel flow to the burner 10. Referring now to FIG. 5, which illustrates the operation of the thermostatic control device of the present invention under the control of a modulating thermostat circuit, when a demand for heat exists, heating coil 88 will become energized and will move lever arm 56 to open port 50 and establish step flow to the main burner 10. The low rate flow to the main burner permits complete ignition without roll-out. Since bimetal 86 cooperates with heating coil 88 to provide a relatively slow acting operator for lever arm 56, full outlet pressure will not be established to the main burner until a sufficient delay time has elapsed to permit complete ignition at the burner; thus, regardless of the degree of electrical energization supplied to heating coil 88, the slow acting bimetal 86 will prevent full outlet pressure from being established until ignition is assured. Subsequent to ignition at the low stepped rate, the thermostatic modulating circuit will apply a varying amount of electrical energization to heating coil 88 in accordance with sensed heat requirements such that the movement of lever arm 56 is correspondingly modulated between the positions shown in FIG. 2, representing the step, and

FIG. 3, representing full outlet pressure. As with the bimetal operated thermostat, when the demand for heat ceases, the step position will be maintained until port 50 is closed whereupon the main fuel flow will be cut off. i 1

The thermostatic heat motor operated control device of the present invention may be additionally utilized in conjunction with an electro-mechanical two-stage ther- I mostat. Such thermostats are well known in the art and typically provide either of two electrical operating potential outputs, the first being a low potential in response to actuation of the first stage of the thermostat, and the second being a relatively high potential indicating that the second stage of the thermostat has been energized and a greater heat input is desired. Thus, in response to the actuation of the first and second stages of the thermostat, low and high voltages, respectively, will be provided in response to demands for heat at low and high rates, respectively. Referring to FIG. 2, when the first stage of the thermostatic assembly is actuated, a low potential will be applied to heating coil 88 of bimetal 86 causing lever 56 to move counterclockwise and open port 50 to establish a step flow through the main diaphragm valve 28 as illustrated in FIG. 6. The step flow will continue until the second stage of the thermostat is actuated to apply a high potential to heating coil 88. With a high potential applied to the heating coil, bimetal 86 becomes heated to its full flow position as illustrated in FIG. 3, such that bypass valve 72 is closed and the main diaphragm valve 28 is moved down to fully open the main fuel flow passage to the burner 10. After the demand for second stage heating ceases, the thermostat will revert to first stage energization only, whereupon the low operating potential will be applied to the heating coil 88 so as to permit the bimetal 86 to cool and open valve 72 to place main valve 28 in its step position shown in FIG. 2. As seen in FIG. 6, the control device is capable of operating cyclically in this manner, and two complete cycles of the system between the first and second stage energization levels of the thermostat assembly 14 are illustrated.

It can therefore be seen that the thermostatic control device of the present invention provides effective step opening of a main diaphragm valve by the sequential action of a bleed valve and a bypass valve under the control of a relatively slow acting heat motor operator formed by a bimetal and a heating coil. Thus, the present invention is simple, yet effective to provide low pressure step flow to a main burner such that ignition will be assured without producing roll-out." Furthermore, the present invention has increased versatility and can be utilized in conjunction with a number of various thermostat assemblies so as to provide stepped operation as well as flow modulation and staged flow control.

Inasmuch as the present invention is subject to many variations, modifications and changes in detail, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a control system for'supplying a fuel flow to burner apparatus, the combination comprising a casing having inlet and outlet means,

differential pressure operated diaphragm valve means mounted in said casing between said inlet and outlet means and operatively moveable between a plurality of flow positions,

said diaphragm valve means cooperating with a wall of said casing to define an operating pressure chamber whereby pressure variations therein cause operation of said diaphragm valve means,

a bleed chamber communicating with said operating pressure chamber and having first and second ports, I

a primary bleed flow passage between said inlet means and said bleed chamber,

a secondary bleed flow passage between said inlet means and said first port,

bleed line means between said second port and said outlet means,

bypass valve means cooperating with said first port to control a bleed flow through said secondary bleed fiow passage into said bleed chamber,

bleed valve means cooperating with said second port to control a bleed flow through said bleed line means from said bleed chamber,

lever means mounted within said bleed chamber and cooperating with said bypass valve means and said bleed valve means and having an off position wherein said bypass valve means is open and said bleed valve means is closed, an on position wherein said bypass valve means is closed and said bleed valve means is open, and a modulating range of positions extending from said off position to said on position wherein said bypass valve is open to a degree dependent upon the particular position of said lever means in said modulating range and said bleed valve means is open,

thermostat means, and

heat motor means operatively connected with said thermostat means and said lever means and responsive to a modulated demand from said thermostat means to effect movement of said lever means between said off, modulating and on positions whereby said diaphragm valve means is moved from an off position to a step flow position and thereafter is modulated between the step flow 3. The invention as recited in claim 2 wherein said bypass valve means is mounted within said bleed chamber adjacent a free end of said lever arm for selective engagement therewith to open and close said first port.

4. The invention as recited in claim 1 wherein said first port comprises an annular seat, and wherein said bypass valve means comprises a bypass valve member disposed on a side of said annular seat, a retaining disc disposed on an other side of said annular seat, and a valve stem concentrically disposed within said annular seat and connecting said bypass valve member with said retaining disc. I

- 5. The invention as recited in claim 4 wherein said bypass valve means further comprises spring means mounted in compression between said other side of said annular seat and said retaining disc whereby said bypass valve means is biased in a direction to close said first port. 7 v

6. The invent on as recited in claim 1 wherein said primary and secondary bleed flow passages each have flow restriction means therein.

7 .The invention as recited in claim 1 wherein said first and second ports comprise annular seats axially displaced from each other and lying in parallel planes. 8. The invention as recited in claim 1 wherein said bypass valve means and said bleed valve means are mounted upstream of said first port and said second port, respectively.

9. The invention as recited in claim 1 wherein said second port comprises an annular seat, and whereih said bleed valve means comprises a flat, flexible bleed valve member mounted on said lever means.

10. The invention as recited in claim 9 wherein said lever means includes a pivotally mounted lever arm exhibiting limited fiexure, and wherein said bleed valve member is mounted upstream of said second port such that a pressure differential is established across said bleed valve means whenever said second port is closed, said pressure differential acting to maintain said second port closed whereby said lever means movesfrom said off position with snap-action.

11. The invention as recited in claim 1 wherein said heat motor means is mounted within said bleed chamber. 

1. In a control system for supplying a fuel flow to burner apparatus, the combination comprising a casing having inlet and outlet means, differential pressure operated diaphragm valve means mounted in said casing betweEn said inlet and outlet means and operatively movable between a plurality of flow positions, said diaphragm valve means cooperating with a wall of said casing to define an operating pressure chamber whereby pressure variations therein cause operation of said diaphragm valve means, a bleed chamber communicating with said operating pressure chamber and having first and second ports, a primary bleed flow passage between said inlet means and said bleed chamber, a secondary bleed flow passage between said inlet means and said first port, bleed line means between said second port and said outlet means, bypass valve means cooperating with said first port to control a bleed flow through said secondary bleed flow passage into said bleed chamber, bleed valve means cooperating with said second port to control a bleed flow through said bleed line means from said bleed chamber, lever means mounted within said bleed chamber and cooperating with said bypass valve means and said bleed valve means and having an off position wherein said bypass valve means is open and said bleed valve means is closed, an on position wherein said bypass valve means is closed and said bleed valve means is open, and a modulating range of positions extending from said off position to said on position wherein said bypass valve is open to a degree dependent upon the particular position of said lever means in said modulating range and said bleed valve means is open, thermostat means, and heat motor means operatively connected with said thermostat means and said lever means and responsive to a modulated demand from said thermostat means to effect movement of said lever means between said off, modulating and on positions whereby said diaphragm valve means is moved from an off position to a step flow position and thereafter is modulated between the step flow position and a full flow position.
 2. The invention as recited in claim 1 wherein said lever means includes a pivotally mounted lever arm.
 3. The invention as recited in claim 2 wherein said bypass valve means is mounted within said bleed chamber adjacent a free end of said lever arm for selective engagement therewith to open and close said first port.
 4. The invention as recited in claim 1 wherein said first port comprises an annular seat, and wherein said bypass valve means comprises a bypass valve member disposed on a side of said annular seat, a retaining disc disposed on an other side of said annular seat, and a valve stem concentrically disposed within said annular seat and connecting said bypass valve member with said retaining disc.
 5. The invention as recited in claim 4 wherein said bypass valve means further comprises spring means mounted in compression between said other side of said annular seat and said retaining disc whereby said bypass valve means is biased in a direction to close said first port.
 6. The invention as recited in claim 1 wherein said primary and secondary bleed flow passages each have flow restriction means therein.
 7. The invention as recited in claim 1 wherein said first and second ports comprise annular seats axially displaced from each other and lying in parallel planes.
 8. The invention as recited in claim 1 wherein said bypass valve means and said bleed valve means are mounted upstream of said first port and said second port, respectively.
 9. The invention as recited in claim 1 wherein said second port comprises an annular seat, and wherein said bleed valve means comprises a flat, flexible bleed valve member mounted on said lever means.
 10. The invention as recited in claim 9 wherein said lever means includes a pivotally mounted lever arm exhibiting limited flexure, and wherein said bleed valve member is mounted upstream of said second port such that a pressure differential is established across said bleed valve means whenever said second port is closed, said pressure differential acting to maintain said second port closed whereby said lever means moves from sAid off position with snap-action.
 11. The invention as recited in claim 1 wherein said heat motor means is mounted within said bleed chamber. 